The present invention relates to a method for detecting alignment patterns in a wafer exposure device, and more particularly to a reduction projection system alignment method suitably adapted for detecting TTL (through the lens) system alignment in a reduction projection exposure device.
In the fabrication of integrated circuits including fine circuit patterns, such as VLSI's, a reduction projection exposure device, in which a reticle pattern reduced to 1/5 or 1/10 using a reduction lens is projected and printed on a wafer, has been frequently employed. When printing the pattern, it must be caused to coincide with a pattern previously formed on the wafer in their position with a high precision. This work is referred to as alignment. This alignment can be carried out with the highest precision by means of a method of detecting the alignment state of a reticle pattern relative to a wafer pattern through a reduction lens (this is referred to as a TTL system). In this case, the reduction lens generally eliminates color aberrations, spherical aberrations, etc. only at a exposure wavelength (monochromatic light such as a g-line, i-line, etc.) so as to provide a high resolution. Therefore, if the alignment pattern detection is carried out using the exposure wavelength or the wavelength in the vicinity thereof, respective alignment pattern images of the wafer and reticle can be detected with the wafer surface held at a focusing position during the exposure. Such a detection system is described in e.g. Japanese Patent Unexamined Publication No. 138,134/82. This system has a disadvantage that resists (exposure agents) in the peripheries of alignment patterns are exposed during the detection.
To obviate this disadvantage, the pattern detection may be carried out with monochromatic light of the wavelength not exposing the resist. In this case, however, it is necessary to correct the degree of out-of-focus of the reduction lens due to its color aberration. For such a detection system, there has been adopted a correction optical system disclosed in Japanese Patent Unexamined Publication No. 110,234/81 or a detection device in which a spatial image resulting from the alignment pattern of the reticle or wafer is employed for the detection, such as disclosed in Japanese Patent Unexamined Publication Nos. 49,910/83 and 112,330/83.
In these detection devices, the alignment patterns (including spatial images) on the wafer and reticle are located in a focusing plane of a pattern detection light through the reduction lens. Thus, no consideration has been taken on some fear of distorting pattern detection waves, reducing the alignment precision or producing any erroneous detection because of the superposition of the alignment patterns of the wafer and reticle.